This invention relates generally to the field of fluid-particle contact and more specifically to a method for operation of the moving beds of radial or horizontal flow fluid-solid contacting devices. More specifically, this invention is related to a method for the contacting of a hot fluid stream with particulate material in a particle bed from which particles are continuously or periodically added and withdrawn.
A wide variety of processes use radial or horizontal flow reactors to effect the contact of a compact bed of particulate matter with a fluid and in particular a gaseous stream. These processes include hydrocarbon conversion, adsorption, and exhaust or flue gas treatment. In most of these processes, contact of the particulate material with the fluid decreases the effectiveness of the particulate material in accomplishing its attendant function. In order to maintain the effectiveness of the process, systems have been developed whereby particulate material is semi-continuously withdrawn from the contacting zone and replaced by fresh particulate material so that the horizontal flow of fluid material will constantly contact a compact bed of particulate material having a required degree of effectiveness. A moving bed system has the advantage of maintaining production while the catalyst is removed or replaced. Typical examples and arrangements for such systems can be found in U.S. Pat. Nos. 3,647,680; 3,692,496; and 3,706,536; the contents of each of which are hereby incorporated by reference. A good example of the way in which moving bed apparatus has been used for the contacting of fluids and solids is found in the field of petroleum and petrochemical processes especially in the field of hydrocarbon conversion reactions. Many hydrocarbon conversion processes can also be effected with a system for continuously moving catalyst particles as a compact column under gravity flow through one or more reactors having a horizontal flow of reactants. One such process is the dehydrogenation of paraffins as shown in U.S. Pat. No. 3,978,150, and another such process is the dehydrocyclodimerization of aliphatic hydrocarbons.
Another well-known hydrocarbon conversion process that uses a radial flow bed for the contact of solid catalyst particles with a vapor phase reactant stream is found in the reforming of naphtha boiling hydrocarbons. This process uses one or more reactors. Typically, the catalyst particles enter the top of a first reactor, flow downwardly as a compact column under gravity flow, and are transported out of the first reactor. In many cases, a second reactor is located either underneath or next to the first reactor. Catalyst particles again move through the second reactor as a compact column under gravity flow. After passing through the second reactor, the catalyst particles may pass through additional reactors before collection and transportation to a regeneration vessel for the restoration of the catalyst particles by the removal of coke and other hydrocarbon by-products that accumulate on the catalyst in the reaction zone.
In the reforming of hydrocarbons using the moving bed system, the reactants typically flow serially through the reactors. The reforming reaction is typically endothermic so the reactant stream is heated before each reactor to supply the necessary heat for the reaction. The reactants flow through each reactor in a substantially horizontal direction through the bed of catalyst. The catalyst particles in each reactor are typically retained between an inlet screen and an outlet screen that together form a vertical bed and allow the passage of vapor through the bed. In most cases the catalyst bed is arranged in an annular form so that the reactants flow radially through the catalyst bed.
Experience has shown that the horizontal flow of reactants through the bed of catalyst can interfere with the gravity flow removal of catalyst particles. This phenomenon is usually referred to as hang-up or pinning and it imposes a constraint on the design and operation of reactors with a horizontal flow of reactants. Catalyst pinning occurs when the frictional forces between catalyst particles and the outlet screen that resist the downward movement of the catalyst particles are greater than the gravitational forces acting to pull the catalyst particles downward. The frictional forces occur when the horizontal flow of vapor passes through the catalyst bed and the outlet screen. When pinning occurs, it traps catalyst particles against the outlet screen of the reactor bed and prevents the downward movement of the pinned catalyst particles. In a simple straight reactor bed, or an annular bed with an inward radial flow of vapors, pinning progresses from the face of the outlet screen and, as the vapor flow through the reactor bed increases, it proceeds out to the outer surface of the bed at which point the bed is described as being 100% pinned. Pinning between the outlet screen and the outer surface occurs when the frictional forces between catalyst particles that resist the downward movement of the catalyst particles are greater than the gravitational forces acting to pull the catalyst particles downward, thereby trapping catalyst particles against pinned catalyst particles. Once pinning has progressed to the outermost portion of the catalyst bed, a second phenomenon called void blowing begins. Void blowing describes the movement of the catalyst bed away from an inlet screen by the forces from the horizontal flow of vapor and the creation of a void between the inlet screen and an outer catalyst boundary. The existence of this void can allow catalyst particles to blow around or churn and create catalyst fines. Void blowing can also occur in an annular catalyst bed when vapor flows radially outward through the bed. With radially outward flow, void blowing occurs when the horizontal flow of vapor creates a void between the inner screen and the inner catalyst boundary. Therefore, high vapor flow can cause void blowing in any type of radial or horizontal flow bed.
The trapping of catalyst particles within a reactor bed that is designed to move continuously causes some catalyst particles to remain in the bed for a longer time than other catalyst particles that still move freely through the bed. As the trapped catalyst particles deactivate and thereby become less effective at promoting the desired catalytic reactions, the reactor bed as a whole exhibits a performance decline, which imposes a direct loss in the production of the desired product. In addition, the production of fines can pose a number of problems in a continuous moving bed design. The presence of catalyst fines increases the pressure drop across the catalyst bed thereby further contributing to the pinning and void blowing problems, can lead to plugging in fine screen surfaces, contributes to greater erosion of the process equipment, and in the case of expensive catalysts imposes a direct catalyst cost on the operation of the system. Further discussion of catalyst fines and the problems imposed thereby can be found in U.S. Pat. No. 3,825,116, which also describes an apparatus and method for fines removal.
Where possible, horizontal or radial flow reactors are designed and operated to avoid process conditions that will lead to pinning and void blowing. This is true in the case of moving bed and non-moving bed designs. Apparatus and methods of operation for avoiding or overcoming pinning and void blowing problems are shown in U.S. Pat. Nos. 4,135,886; 4,141,690; 4,250,018; and 4,567,023, the contents of each of which are incorporated herein by reference. To avoid process conditions that lead to pinning, it has been the practice for many years to operate reactors of continuous and semi-continuous moving bed designs by maintaining the flow of vapor through the bed of catalyst at a rate that is below the rate that will pin catalyst when the bed is stagnant. This rate is referred to herein as the stagnant bed pinning flow rate.
As explained in further detail in the detailed description below, the stagnant bed pinning flow rate is the fluid rate that prevents at least a portion of the particles in a bed, which is initially stagnant, from moving downward when particles are withdrawn from the bottom of the bed. In the past, the stagnant bed pinning flow rate has been estimated using a theoretical analysis of the mechanics within the stagnant bed of particles. A suitable analysis is described in the article written by J. C. Ginestra et al. at pp. 121-124 in Ind. Eng. Chem. Fundam. 1985, 24. The inputs to this analysis are the physical properties of the particles; the condition of the particle bed; the geometry of the particle bed and of the screens and walls retaining the bed; the physical properties of the screens and walls, if any, retaining the bed; the physical properties of the fluid; and the operating conditions of the bed. The condition of the particle bed takes into account the solid fraction of the particle bed, the particle-screen static friction factor, and the particle-particle static friction factor. Confirmation of this estimate of the stagnant bed pinning flow rate can be obtained by experiment. The experimental apparatus is a vertically-extended bed of particles between an inlet screen and an outlet screen, with an inlet at the top of the bed and an outlet at the bottom of the bed for downward flow of particles. The apparatus also has a fluid inlet and a fluid outlet for cross-flow of a fluid. While the particle outlet is closed, the particle bed is formed by introducing particles through the particle inlet in the same manner as particles are introduced through the inlet when particles are flowing downward through the bed. Only a short time after loading in order to ensure that the solid fraction of the particle bed is essentially the same as when the particles are loaded, the fluid flow rate through the bed of particles is started at a relatively high rate such that, once downward flow of particles begins, a substantial portion (i.e., about 25-50%) of the particles within the bed is pinned. Then, the particle inlet and outlet are opened, so that particles flow downward through the bed. Next, the flow rate is reduced stepwise, with each reduction in flow unpinning some of the particles that had been pinned, until the final downward step in flow rate results in no pinning of any of the particles. The stagnant bed pinning flow rate can be determined by averaging the penultimate and final flow rates. The precision of the measurement of the stagnant bed pinning flow rate can be improved by decreasing the step size between the penultimate and final flow rates.
Many moving bed design reactors in commercial plants around the world have operated for years and even decades at vapor rates below the stagnant bed pinning flow rate described in the preceding paragraph and thereby have successfully avoided any pinning problems. Despite thousands of successful plant-years of operation, catalyst pinning, although rare, can occasionally occur in a radial flow reactor of the continuous or semi-continuous moving bed design. When pinning does occur, a short procedure is typically used to xe2x80x9cunpinxe2x80x9d any pinned catalyst. First, the vapor flow rate is decreased significantly below, i.e., typically at least 10-50% below, the stagnant bed pinning flow rate, and then the flow rate is increased to a rate which is less than the stagnant bed pinning flow rate. The catalyst withdrawal rate may be stopped or decreased, either before, simultaneously with, or after the reduction in vapor rate. If the catalyst withdrawal is stopped or decreased, then it is usually restarted or increased prior to increasing the vapor flow rate. In cases of severe pinning where this short procedure is unsuccessful, the vapor flow is stopped and the pinned catalyst is manually removed from the bed.
Occasionally, commercial reactor beds that are designed to move continuously stop moving and come temporarily to rest. This happens intentionally when the reactor bed is designed for semi-continuous catalyst withdrawal. Depending on the design and operation of the commercial plant, these periods of time at rest can be in the range of from as low as 1-2 minutes to as high as 6-12 months, but they are commonly in the range between 10 minutes and 1 hour. When catalyst flow is resumed, catalyst in these reactor beds does not become pinned, as evidenced by the absence of any symptoms of pinning.
Methods of operation for increasing the vapor flow rate in moving bed processes while avoiding pinning problems are sought.
In a surprising discovery, it has now been recognized for the first time that, when a fluid flows transversely through a moving particle bed and is recovered from a perforated outlet partition, the duration of time during which the particle bed has been at rest can significantly affect whether or not particles pin against the outlet partition. Although this effect has been observed in beds containing cohesionless particles, this discovery is believed to be also applicable to particles that exhibit some cohesion. Exploiting this unexpected discovery, this invention is a process for passing a fluid through a bed of particles at a rate greater than the previously-defined stagnant bed pinning flow rate, which is the upper limit on fluid flow rate in the prior art processes. By keeping the particles moving or by decreasing, or keeping at a minimum, their time at rest, this invention permits fluid to pass through the particle bed at flow rates far above the stagnant bed pinning flow rate. If the particles do come to rest while the fluid flow rate exceeds the stagnant bed pinning flow rate, then according to this invention the duration of the time at rest is limited either to a specified period of time or to a period of time during which the solid fraction increases by only a specified amount. Of the particles in the bed, those that should move or should be prevented from not moving in accord with this invention are those particles that are closest to the outlet screen, since pinning generally progresses from the outlet screen. However, a process in which all of the particles in the bed keep moving is within the scope of this invention. In any event, by allowing particle beds to operate at fluid throughputs outside the constraints imposed by the prior art, this invention significantly increases the efficiency and profitability of processes that use new and existing particle beds.
The present discoveries show that, for a given particulate material maintained in a bed of a given geometry, the pinning flow rate when the bed is moving (i.e., the moving bed pinning flow rate) is significantly greater than the stagnant bed pinning flow rate. As explained in further detail in the detailed description below, the moving bed pinning flow rate is the fluid rate that prevents some of the particles in a bed, which is initially moving, from moving downward when particles are withdrawn from the bottom of the bed. Experimental results show that the moving bed pinning flow rate can be up to 60% higher than stagnant bed pinning flow rates, depending on the bed configuration, particulate material, and process conditions. This unexpected discovery led to the present invention, which is a dramatic breakthrough process in the sense that this invention shatters preexisting notions about processing barriers. This invention surpasses hydraulic constraints that had previously been considered insurmountable. In contrast to the prior art which viewed the stagnant bed pinning flow rate as a limit on the flow rate of reactants through a bed of particulate material, this invention allows for flow rates of reactants through the bed that exceed the stagnant bed pinning rate and which approach the moving bed pinning flow rate. In theory, by using this invention, the flow rate of reactants through the bed can be increased by a factor equal to the ratio of the moving bed pinning flow rate to the stagnant bed pinning flow rate. In practice, the reactant flow rate will be increased by less than that factor, and will typically increase in the range of about 15% to about 60% of the reactant flow rate in the prior art processes. Nevertheless the economic and technical benefits that accrue from even a 1% increase in the flow rate of reactants through an existing particular bed are enormous.
Without being bound by any particular theory, it is believed that the explanation why the fluid flow rate can be increased while avoiding pinning problems is that the particle-screen screen friction is significantly less when the particulate material is moving relative to the screen than when the particulate material is stagnant relative to the screen, and therefore pinning decreases. The particle-screen friction is caused by particles contacting the screen and is expressed by the particle-screen friction factor. The particle-screen kinematic friction factor expresses the friction caused by particles moving or sliding against the screen, whereas the particle-screen static friction factor expresses the friction between particles and the screen when the particles and the screen are not moving relative to each other. The particle-screen friction is significantly lower not only because the solid fraction in the bed decreases but also because the particle-screen kinematic friction factor itself decreases. In addition to the belief that the particle-screen kinematic friction factor decreases relative to the particle-screen static friction factor, it is also believed that the particle-screen kinematic friction factor is only very weakly dependent on the flow rate of the particulate material. So, as long as the particulate material is moving or sliding to any extent along the screen, it is believed that the particle-screen kinematic friction factor remains significantly less than the particle-screen static friction factor. Thus, the critical characteristic of moving or sliding particles along the screen in this invention is not how fast they are moving, but that they are in fact moving.
Similarly, it is also believed, without being bound by any particular theory, that the particle-particle kinematic friction factor is significantly less than the particle-particle static friction factor, and also that the particle-particle kinematic friction factor is only very weakly dependent on the flow rate of the particulate material across the screen. The particle-particle static friction factor expresses the friction between solid particles that are not moving against each other, and the particle-particle kinematic friction factor expresses the friction caused by particles flowing against each other. While the particle-screen friction is an important factor in the onset of pinning at the outlet screen, the particle-particle friction is an important factor in determining the extent and shape of the volume of pinned particles in the bed.
It has also now been recognized, as a result of this invention, that for many new moving-bed commercial processes, a semi-continuous moving bed design in which the bed is at rest for long periods of time may not be optimum from a process economics viewpoint. In most cases, the capital that would be invested in equipment and control systems for a semi-continuous moving bed design would yield a much greater return if invested instead on the equipment and systems for a continuous moving bed design, because the continuous moving bed design allows for a significant increase in processing throughput through an existing particle bed process, which leads to greater profitability. Investing capital in a revamp of an existing semi-continuous moving bed processes into a continuous process will also yield a high return on investment.
In addition, this invention provides a method for unpinning particles in a particle bed. First, and optionally, the addition and withdrawal of particles to and from the bed is stopped. Then, the fluid flow rate is decreased to a rate that is less than the stagnant bed pinning flow rate of the bed. Finally, if the particle flow to and from the bed was stopped, then the particle flow is resumed.
This invention also provides a method of raising the fluid flow rate through the bed to a rate that is more than the stagnant bed pinning flow rate. Initially, the fluid flow rate is less than the stagnant bed pinning flow rate, and then the addition and withdrawal of particles to and from the bed is begun. Finally, the fluid flow rate is increased to a rate above the stagnant bed pinning flow rate.
This invention stands in stark contrast to prior art fluid-particle contacting processes. Embodiments of this invention that keep the particles moving differ from two prior art processes, namely those processes where the particles move only when the fluid rate is less than the stagnant bed pinning flow rate and those processes where the particles are not moving when the fluid rate exceeds the stagnant bed pinning flow rate. Embodiments of this invention that regulate the time during which the particles are not moving when the fluid rate is greater than the stagnant bed pinning flow rate differ from the prior art process in which the particles do not move at all while the fluid rate is greater than the stagnant bed pinning flow rate. Such embodiments are also different from the prior art process in which the particles move at times and do not move at other times, but the fluid rate is less than the stagnant bed pinning flow rate.
In a broad embodiment, this invention is a process for passing a fluid through a bed of particulate material. A particulate material is maintained in a vertically extended bed having a fluid inlet face. The bed is maintained between the fluid inlet face and an outlet partition that has a perforated section extending over at least part of its length. The size of the perforations retains the particulate material while permitting fluid flow through the perforations. The particulate material is withdrawn from the bottom of the bed. An inlet fluid passes to the fluid inlet face and transversely through the bed. An outlet fluid is recovered from the perforated section of the outlet partition at an operating flow rate that is not less than a stagnant bed pinning flow rate. In another embodiment, the particulate material in the bed is prevented from coming to rest for a period of time.
Other objects, embodiments and details of this invention are disclosed in the following detailed description.
The article written by J. W. Carson et al. at pp. 78-90 in Chemical Engineering, April 1994, which is incorporated herein by reference and hereinafter referred to as xe2x80x9cCarson,xe2x80x9d teaches that, in the field of bulk solids handling in bins, hoppers, and feeders, the time that a bulk solid is stored at rest can affect the flow behavior of the bulk solid, since time of storage at rest can determine in part the cohesiveness and the frictional properties of a bulk solid. Carson teaches that, when a material resides in a bin or hopper for a period without moving, it can become more cohesive and difficult to handle, and that such cohesion may be caused by settling and compaction, crystallization, chemical reactions, and adhesive bonding.
The article written by J. Marinelli et al. at pp. 22-28 in Chemical Engineering Progress, May 1992, which is incorporated herein by reference and hereinafter referred to as xe2x80x9cMarinelli,xe2x80x9d teaches that the time that a bulk material is at rest can influence the wall friction and can affect the determination of the size of the outlet of a bin or hopper.
The article written by J. C. Ginestra et al. at pp. 121-124 in Ind. Eng. Chem. Fundam. 1985, 24, analyzes the mechanics of pinning of a bed of particles in a vertical channel by a cross flow of gas.
The paper written by S. B. Savage, which is published at pp. 261-282, in the book entitled Mechanics of Granular Materialsxe2x80x94New Models and Constitutive Relations, edited by J. T. Jenkins et al., Studies in Applied Mechanics 7, Elsevier Science Publishing Co., Inc. New York, 1982, reviews and discusses models that have been applied to predict the flows of dry, cohesionless granular materials down inclined surfaces.
The paper written by H. J. Jaeger et al. at pp. 1523-1531 in Science, Vol. 255, Mar. 20, 1992, describes inhomogeneity, maximum and minimum packing densities, and stress-carrying networks in particle beds.
The article entitled, xe2x80x9cHidden in the Hopper: A Secret of Physics,xe2x80x9d written by James Glanz and published in The New York Times, Jan. 9, 2001, section F, page 3, column 1, describes the difficulties that research physicists have had in devising a theory of xe2x80x9cjammingxe2x80x9d of granular materials in hoppers that takes into account the xe2x80x9cnasty realitiesxe2x80x9d of industrial practice, such as the effects of friction between particles of the granular material, humidity, other atmospheric conditions, and three-dimensional flow patterns.